At the heart of the typical central air conditioning system is a combination of electro-mechanical elements that work together on a refrigerant fluid, e.g., one of the Freon.TM. compounds, according to a refrigeration cycle. Typically, the Freon vapor is compressed by an electrically driven compressor and the compressed vapor is cooled by being passed through a heat exchanger, commonly known as a condenser, after which it is throttled and passed through a second heat exchanger where it picks up heat from air within the building. The refrigerant is then returned to the compressor to undergo the cycle once again.
Most conventional heat exchangers generally consist of a nest of tubes made of a thermally highly conductive metal like copper, to which are attached numerous thin metallic fins which conduct away heat from the tubing to transfer it to air flow directed between and over the fins. A motor driven fan typically directs air flow through the fins surrounding the nested tubes. To reduce both the cost of the structure and the power requirements of the fan directing the air flow through the heat exchanger, it is important to maximize the rate at which the refrigerant fluid flowing through the tubes transfers heat to or from the air flowing past the tubes and between the fins, i.e., the "air-side heat transfer."
One solution is to increase the total area of the fins by increasing the number of fins to obtain increased transfer of heat by forced convection to the air flowing therebetween. This, however, soon diminishes the size of the passages between the fins through which the air must flow and would require a more powerful fan to provide the pressure difference to force the desired amount of air flow through the fins. A second alternative is to provide reasonably spaced apart fins having a waffle-like or undulating configuration to increase the area exposed to the air flow. Unfortunately, with this latter solution, a problem arises in the growth of velocity and heat transfer boundary layers which very soon diminish the amount of heat transfer that can take place between the flowing air and the fin surfaces. In recognition of this problem, designers of heat exchangers have focused on techniques to inhibit the growth of velocity and heat transfer boundary layers without significantly increasing the overall pressure difference required to obtain the desired flow of air through the tube and fin assembly.
Heat transfer by conduction must first occur between the surface of the refrigerant carrying tubing and the fins and, thereafter, by convection from the fin surfaces to the air flowing between the fins. There is also a direct transfer of heat from the surface of the tubing by convection to the air flowing past the tubing, but this generally amounts to only a small fraction of the overall heat transfer. It should also be remembered that there are certain limits of material strength and manufacturing limitations which constrain a designer who seeks to shape the fins to maximize the transfer. Examples of patented solutions to the above discussed problems are contained in the following.
U.S. Pat. No. 2,079,032, to Opitz, discloses corrugated edges on the fins to strengthen the fins, as well as fin portions that form substantial angles at the tube collars where the tubes pass through the fins with the focus being on the corrugated fin construction to strengthen the assembled heat exchanger against crushing forces. U.S. Pat. No. 4,480,684, to Onishi et al, teaches the use of offset tube collars in the fins, with the fins themselves lanced in bridge-like formations each of which is parallel to the fin corrugation thereat. U.S. Pat. No. 4,469,168, to Itoh et al, discloses enhanced heat transfer fins that have louvers inclined at a small angle to the direction of flow of the cooling air in a direction opposite to that of the inclination of the fins locally. U.S. Pat. No. 4,469,167, also to Itoh et al, discloses enhanced fins having louvers offset above and below the plane of the fin and inclined at a predetermined angle to the direction of the air flow past the fins. U.S. Pat. No. 4,300,629, to Hatada et al, discloses enhanced fins having peaked bridge-like portions. All of the above mentioned patents, and others of a like nature, offer solutions intended to increase the turbulence in the air flow to inhibit the growth of velocity and heat transfer boundary layers on the fin surface, thereby to ensure a higher efficiency in the heat exchanger.
Because the turbulent air flow through the heat exchanger does not lend itself to satisfactory theoretical analysis, there is need for empirical improvement in heat exchanger fin designs.